1. Field of the Invention
This invention relates generally to control apparatus for delivering solar energy from a heliostatic field to a receiver and, more particularly, to control systems that automatically correct for error between the heliostat's command reference system and its inertial reference system.
2. Description of the Prior Art
In order for a heliostat field to deliver the required sun energy to its receiver, the heliostats require a beam pointing control accuracy of less than 1.5 mr rms over the day. There are many different classes of errors that can result in a beam pointing error. One such class of errors is track alignment error which produces a difference between the inertial reference system and the heliostat command reference system. This error may be produced in the original construction of the foundation, subsequent foundation settling, movement resulting from high winds, earthquakes, etc.
The aimpoint may be altered by improper centering of the foundation. Also, if the foundation, pedestal, and drive are not installed vertically, the heliostat azimuth rotational point at the top of the pedestal will be displaced from the desired aimpoint.
Azimuth rotational axis tilt results from misalignment between the actual azimuth rotational axis and the local vertical. The verticality of the foundation, tilt of the pedestal, or a misalignment of azimuth drive, etc., are some of the things which cause rotational tilt errors.
All position commands for the heliostat are calculated with respect to the local horizontal plane and a south reference line. If the heliostat reference system is not aligned to this system, then a beam error will result.
A number of alignment measuring methods have been investigated and have been proven to be unable to meet the accuracy requirement or were very costly because of extensive field labor required to correct for misalignment in the system. Time consuming methods require resurveying the heliostat site to minimize alignment errors, taking physical heliostat measurements in the field, and making calculations in order to determine the track alignment error source values for a single heliostat. It would be desirable if there were provided an efficient, time and cost saving method in which to set up a heliostat for tracking operations having the additional capability to automatically reset the heliostat track alignment which may change as a result of continuous operation over time.